The invention relates to novel bisphenylthio compounds and their use as lubricants.
In order to reduce the wear and the energy loss due to friction, machine bearings and gears are known to be provided with a lubricant which, during operation, permits as complete separation as possible of the solid bodies moving against one another. The lubricants may be divided into lubricating liquids and lubricating greases. Conventional lubricating liquids in use are mineral oils obtained from petroleum, synthetic oils, such as polyalkylene glycols, ethylene polysulphides, ester oils or phosphoric acid esters, and silicone oils (Ullmann""s Encyclopedia of Industrial Chemistry, 5th Edition, Vol. A15, page 423 et seq., VCH Weinheim 1990). Mesogenic liquids, i.e. liquids which form thermotropic liquid crystalline phases and can be; transformed from a low-viscosity into a high-viscosity phase, and vice versa, in the lubricating gap (U.S. Pat. No. 5,160,451) are also known. In general, a liquid is referred to as mesogenic if it can form one or more liquid crystalline phases under specific conditions (pressure, temperature, shearing, surface interactions). The relationships between chemical structure and the arrangement of the molecules in such phases and the temperature range in which such phases can occur is known (e.g. H. Kelker, R. Hatz, Handbook of Liquid Crystals, Verlag Chemie, Weinheim 1980; C. Destrade et al., Mol. Cryst. Liq. Cryst. Vol. 106, 121 (1984). Lubrication with lubricating greases is widely used. These lubricating greases consist of a lubricating liquid and, dispersed therein in fine form, a solid, the so-called thickener which has only a slight effect on the tribological properties and is primarily a reservoir for the lubricating liquid. In addition, very low-viscosity organic substances, such as fuels for spark-ignition engines (petrol) and diesel engines (diesel fuel), can also be considered as lubricants, which is clear from the known effect of the fuel on the wear of nozzle valves of fuel injection nozzles. Solutions and emulsions of organic compounds in water also serve as lubricants, in particular as so-called cooling lubricants in the machining of metallic workpieces.
The energy losses and wear phenomena occurring in a friction pairingxe2x80x94these are generally solid bodies separated by a lubrication gap and moving against one anotherxe2x80x94depend in a complex manner on the material of the machine element itself, on the properties of the lubricating oil, such as its viscosity and its interactions with the material, and on the pressure and velocity conditions. Continuous supporting films, as occur, for example, in the hydrodynamic region of sliding bearing or in the elastohydrodynamic region of roller bearings, are advantageous. High frictional losses, characterized by the generally customary coefficients of friction, and the wear phenomena generally correlated with them occur particularly in sliding bearings in the case of so-called boundary friction and mixed friction (cf. Ullmann""s Encycl.).
In addition to the ethylene polysulphides, inter alia hexakis-, pentakis- and tetrakis[alkylthio]benzenes are known as sulphur-containing lubricants (DE 196 11 466). The latter compounds are distinguished by particularly good lubricating properties. Owing to their tendency to form discotic phases under atmospheric pressure at temperatures as low as in the range from 0 to xe2x88x9210xc2x0 C., and in particular such phases which are characterized by a particularly high degree of ordering of the molecules and very high viscosities, these lubricants are not suitable for machine bearings which have to be supplied with a liquid medium at temperatures down to xe2x88x9240xc2x0 C. by a continuous flow process.
It was an object of the invention to provide sulphur compounds which have the advantageous frictional and wear properties of the above-mentioned alkylthiobenzenes but, even at low temperatures, exhibit no transition to a liquid crystalline phase disadvantageous for the flowability.
The object was achieved by providing the compounds, according to the invention, of the general formula I 
in which
X1 to X5 in each case independently of one another represents H, Cl, F, OH, SH, R, xe2x80x94OR and xe2x80x94SR, R being an organic radical having 1 to 18 C atoms which may be an alkyl or alkenyl radical which may also contain xe2x80x94Oxe2x80x94 or xe2x80x94Sxe2x80x94 instead of one to four nonneighbouring CH2 groups and may also be monosubstituted or polysubstituted by xe2x80x94OH, xe2x80x94SH or halogen, or a phenyl, cyclohexyl or cyclohexylmethyl radical which is unsubstituted or substituted by one to 3 alkyl groups having a total of 1 to 9 C atoms, in which in each case a CH2 group may be replaced by xe2x80x94Oxe2x80x94, with the proviso that at least 3 of the radicals X1 to X5 are xe2x80x94SR,
Y1 to Y5 in each case independently of one another have the meaning stated for X1 to X5 and
z denotes an xcex1,xcfx89-alkylene group having 2 to 18 C atoms in which one to 3 CH2 groups may also be replaced by xe2x80x94Oxe2x80x94 or xe2x80x94Sxe2x80x94, or a 1,4-cyclohexanedimethyl group 
xe2x80x83or a single bond.
It was found that the compounds according to the invention are suitable as lubricants which permit particularly small frictional losses in friction pairings and surprisingly are flowable at low temperatures.
The molecules of the compounds of the formula I contain two phenyl rings which are linked via a bridge containing at least two S atoms. Altogether, at least 8 S atoms are contained in the molecule, each phenyl ring being linked to at least 4 S atoms. Preferred compounds are those in which both phenyl groups are each linked to at least 5 S atoms, particularly preferably those in which both phenyl groups are each linked to 6 S atoms.
The substituents X1 to X5 and, in each case independently of one another, Y1 to Y5 are preferably xe2x80x94SR, xe2x80x94H or xe2x80x94SH. The radical R preferably designates an alkyl radical having 1 to 18 C atoms, in which one to four nonneighbouring CH2 groups may also be replaced by xe2x80x94Oxe2x80x94 or xe2x80x94Sxe2x80x94 and which may be substituted by xe2x80x94OH or xe2x80x94SH. Straight alkyl chains are particularly preferred, very particularly those having 6 to 12 C atoms.
The group Z preferably contains 2 to 10 C atoms. Particularly preferred here are straight-chain xcex1,xcfx89-alkylene groups. The 1,4-clohexanedimethyl group is also preferred.
The formulae Ia to If are examples of compounds according to the invention: 
The preparation of the compounds of the formula I is carried out by generally known methods of synthetic chemistry, as described, for example, in R. C. Larock, Comprehensive Organic Transformations, VCH Publishers, Inc. (1989) and in Houben-Weyl, Methoden der Organischen Chemie [Methods of Organic Chemistry], Georg-Thieme Verlag, Stuttgart.
Compounds of the formula I are preferably prepared by first producing, from a mixture of the thiols RSH and HSxe2x80x94Zxe2x80x94SH, in an aprotic solvent, such as N-methyl-2-pyrrolidinone, N,N-dimethylformamide or tetramethyleneglycol dimethyl ether, by reaction with sodium amide or sodium hydride, the corresponding sodium thiolates, which are then converted into mixtures of compounds of the formula I by addition of halobenzenes, such as hexa-, penta- or tetrachlorobenzene, or the corresponding fluorine compounds, in a substitution reaction at temperatures between 50xc2x0 C. and 180xc2x0 C.
If mixtures of the thiols RSH, such as, for example, mixtures selected from the compounds octadecylthiol, hexadecylthiol, tetradecylthiol, dodecylthiol, decylthiol, nonylthiol, octylthiol, heptylthiol, hexylthiol, pentylthiol, butylthiol, propylthiol, ethylthiol and methylthiol, are used, mixtures of numerous compounds of the formula I with a randomly distributed arrangement of the various substituents RS are formed. By selecting the alkyl chain length of the thiols used and their proportions, it is possible to influence the viscosity and flow behaviour at low temperatures.
If, in the reactions mentioned, less than the stoichiometric amount of metal thiolates is chosen, compounds of the formula I having halogen substituents can be obtained.
The introduction of the xe2x80x94SZSxe2x80x94 bridge can also be effected by reaction of phenylthiols with dihaloalkanes in aprotic solvents with the addition of bases, e.g. 
Preferred dihalo compounds are 1,4-dibromobutane, 1,6-dibromohexane, 1,8-dibromooctane, 1,12-dibromododecane and 1,4-bis[bromomethyl]cyclohexane. By known oxidation reactions, substituted diphenyl disulphides of the formula I (Z is a single bond) can be obtained from the above-mentioned phenylthiols.
After the substitution reaction is complete and the solvent has been distilled off, dilute hydrochloric acid is added to the distillation residue. The organic phase which separates from the aqueous phase contains compounds of the formula I in addition to compounds of the formula II in which the radicals X1 to X5 have the above-mentioned meanings. 
The additional substituent X6 may have one of these meanings may be xe2x80x94SZSH and, where Z is an alkylene group, also xe2x80x94SZCl or xe2x80x94SZBr. In addition, a small amount of compounds of the formula III are formed, which have the following general structural formula 
In formula III, the middle phenyl ring can be substituted by the two bridge groups in the ortho, meta or para position. Groups U1 to U4 have the meaning as stated above under X1 to X5.
The compounds of the general formulae II and III are as a rule not removed from the mixture for the intended use as a lubricant. Rather, it may be regarded as a fortunate circumstance that these substances also have advantageous lubricating properties. However, the compounds of the formula I can be isolated by chromatographic methods.
The lubricant according to the invention contains 1 to 100, preferably 1 to 60 and very preferably 5 to 40, percent by mass of compounds of the formula I.
The content of compounds of the formula I in the lubricant according to the invention is expediently determined by the generally known method of gel permeation chromatography (GPC). For identification of the compounds, the known field desorption mass spectroscopy (FD-MS) is advantageously used. It is possible to carry out an assay using this mass spectroscopic method alone, because only the masses of the molecular ions are indicated in the spectra. In this case, the use of calibration substances is necessary.
It was found that the compounds, according to the invention, of the formula I are suitable as lubricants. Compared with conventional lubricants, they permit substantially lower frictional losses in gears and bearings.
The lubricants to which the invention relates may contain exclusively compounds of the formula I. As a rule, however, compounds of the formulae II and III and further components are present. Such components may be, inter alia, antioxidants, such as derivatives of 2,6-di-tert-butylphenol, high-pressure additives, such as zinc dialkyl dithiophosphates, friction-reducing substances, light stabilizers, emulsifiers or demulsifiers. However, organic compounds for varying the viscosity, such as compounds whose molecules contain benzene or naphthalene nuclei polysubstituted by alkyl groups, may also be present. The invention also relates to those lubricants whose main proportion (up to 99 percent by mass) comprise conventional synthetic or mineral oils (cf. Ullmann""s Encycl.). If the lubricant according to the invention is a lubricating oil, all components are present in molecular disperse form in the homogeneous liquid.
Oil-like lubricants can be converted in a generally known manner (cf. Ullmann""s Encycl.) into a lubricating grease by adding thickeners which are not present in molecular disperse form. Particularly suitable thickeners are lithium 12-hydroxystearate and polytetrafluoroethylene powders (e.g. 5 xcexcm microteflon powder, Dr. Tillwich GmbH, Horb). The present invention also includes among the thickeners polymers which serve for the formation of gels, such as, for example, so-called side-chain polymers (H. Ringsdorf et al., Angew. Chem. 101, 934 (1989) and literature cited there), as well as inorganic solid additives, such as molybdenum sulphide or graphite. Such lubricant greases which contain one or more compounds of the formula I and which may contain up to 35% of such thickeners are to be regarded here as lubricants according to the invention.
The invention also relates to so-called cooling lubricants, which are emulsions of organic lubricating oils in aqueous solutions. That fraction of a cooling lubricant which can be isolated as a homogeneous nonaqueous phase from the emulsion by known methods of demulsification, such as ultracentrifuging, salting out or addition of demulsifiers, is regarded as a lubricant according to the invention.
The lubricants according to the invention are distinguished by good continuous flow behaviour at low temperatures. This is advantageous for the trouble-free operation of machine bearings and in particular of gears. In order to assess the continuous flow behaviour of lubricants at low temperatures, the so-called channelling test (Federal Test Method Std. No. 791C, USA) is used: about 600 ml of lubricants are cooled for 18 to 20 hours at the desired test temperature in a container having a diameter of 90 mm and a height of 100 mm. A 20 mm wide furrow is then drawn through the sample with a sheet steel strip (230xc3x9720 mm, thickness 3 mm). If the lubricant flows back within 10 seconds so that the bottom of the container is covered again, the lubricant has flowability adequate for gears.